This invention relates to sealing glass compositions and more particularly to nondevitrifying sealing glass compositions and to a method for sealing together the component parts of television picture tubes using those compositions.
In the fabrication of television picture tubes the faceplate of the picture tube must be hermetically sealed to the tube funnel. Normally, devitrifying sealing glasses are used for this purpose where a devitrifying glass is one which crystallizes during the sealing operation. In contrast, a non-devitrifying (or vitreous) glass does not form crystals during the sealing cycle. Instead, as a vitreous glass is cooled, the viscosity of the liquid increases steadily and the glass can be considered a disordered amorphous solid or a supercooled liquid. Devitrifying glasses have been used because they were thought necessary to insure a hard, rigid bond that would maintain its integrity during subsequent heating steps in the tube fabrication process. The use of devitrifying glasses, however, introduces difficult processing problems such as requirements for rigid temperature-time controls during sealing. With sealing glasses that crystallize, the rate of heating is extremely important. If the rate of heating is too slow, for example, the glass can devitrify before a good bond is formed with the glass pieces being sealed together. The maximum sealing temperature must also be closely controlled to insure proper crystallization. Additionally, since the degree of crystallization depends upon time, the pieces being sealed together must be held at the sealing temperature for long time periods, over one-half hour in many cases, to allow the devitrification reaction to reach completion. Such long sealing times are expensive because of the amount of equipment needed for a given throughput, and because of the energy required. Finally, the use of devitrifying glasses makes the reworking of defective seals almost impossible; once the glass has crystallized, temperatures much higher than the original sealing temperature are required to remelt the seal.
In view of the foregoing, it should now be understood that it would be desirable to provide improved sealing glass compositions that would solve the above and other problems.
Accordingly, one of the objects of the present invention is to provide a nondevitrifying (vitreous) solder glass/ceramic mixture suitable for the sealing together of television picture tube components.
Other vitreous sealing glass compositions have previously been disclosed, such as the composition disclosed in U.S. Pat. No. 3,964,920. This sealing glass composition, however, has been formulated for the specific application of sealing ceramic packages for semiconductor devices and is not suitable for the present use. It is, therefore, a further object of the invention to provide a solder glass/ceramic mixture which has the proper expansion coefficient, strength, sealing temperature, and short required sealing time for the specific application of sealing television picture tubes.
It is still a further object of the invention to provide a method for sealing together the component parts of a television picture tube.